Bridged power amplifiers have a particularly simple power supply format in that only a two terminal supply is required. Such amplifiers offer high power, such as in the tens of kilowatts with wide bandwidth and very low distortion and noise which makes them nearly ideal for precision amplification applications such as gradient amplifiers for magnetic resonance imaging. These amplifiers have though an inherent deficiency in that they are of low power efficiency. Such low or reduced efficiency results in large amounts of semi-conductor materials being used for pass devices and large heat sinks being used to receive the generated and wasted heat due to amplifier operation.
Another type of an amplifier, the switch-mode pulse width modulation (PWM) amplifier, offers improved power efficiency but suffers greatly from poor bandwidth and poor fidelity. PWM bandwidth is typically an order of magnitude less than needed for many applications. Operating with switch frequencies in the megaHertz range in order to attain the necessary bandwidth results in low efficiency and is, therefore, not practical. The use of megaHertz operating frequencies is especially difficult and costly if the output power measures in the tens of kilowatts.
It is known in the practiced art to use two separate PWM buck mode power supplies to power the traditional totem-pole topology (non-bridge) linear amplifier. In addition to requiring two PWM supplies, whereas bridge designs require only one supply, the designs have required either an excessive or inefficient operating efficiency and/or precognition of the amplifier's input signal to allow the slowly responding power supply to get a head start on producing the necessary supply voltage transients. The only method of such precognition that is feasible for many such systems is to delay the main amplified signal by a multiple of milliseconds which is the response time needed by a slow PWM supply. For many uses, such delays of the amplified signal are not allowed nor desirable. In attempting to make a PWM buck supply faster, typically the output voltage ripple from the converter is compromised, and at low operating currents, the output ripple voltage contains large amounts of switching subharmonics as cycle-skipping behavior is manifested by the converter. The missing of cycles is common when a buck converter operates at near zero duty cycle to produce small output currents.
In the use of bridge linear power amplifiers, the application of multi-level signal tracking power supplies for the purpose of powering the amplifiers, such as described in U.S. Pat. Nos. 4,788,452 and 5,045,990, are capable of providing amplifier system efficiencies which are much improved over the basic Class B operation with fixed DC supplies. Therefore, in the concept of the following described invention, a multiple of separate PWM buck mode power supplies are utilized in conjunction with a bridge linear power amplifier to produce and provide a high-efficiency fast precision amplification system which does not suffer from the deficiencies referred to above with respect to the heretofore utilized PWM and bridge linear power amplifiers.